Just seen one of these for sale and was curious of how the trigger mechanism works.
Blurry pic

If that's the LMG system, there are patent drawings out there.  In the LMG system, basically, there's a notch on the hammer and on the bolt carrier, such that the hammer holds the bolt carrier back when it's cocked.

As the trigger is pressed, a cam on the trigger acts on the leg of the auto sear, causing it to cam the hammer down slightly, releasing the bolt carrier, then slipping off the leg of the auto sear.

Bolt carrier flies forward, trigger is pressed, so the hammer is caught by auto sear instead of the trigger sear as normal.

Bolt carrier approaches battery and trips auto sear, releasing hammer to fly forward and strike the firing pin.

But, that doesn't look like the LMG system, nor does it look like the slam-fire open bolt firing port weapon (FPW) fire control group, so I think it might be the tactical innovations open-bolt system for their proprietary upper.  If so, I'm not sure how that system works, since I've never seen or used one.

How about the LWRC system that is open bolt in full auto and closed bolt in semi. I would love to take a look at that system.

EDIT: that looks like a kit I saw on an AM15

The LWRC M6A4 (IAR submission) requires modification of the lower. They need to submit it to the Tech. Branch for a determination of whether the lower or trigger pack is the registered part. If it's determined to be the lower I'll be the first to have one.

This is the Colt LMG system:

I thought the colt open bolt LMG had a striker that slides back and forth inside the bolt carrier

I think the striker is on the FPW system that Circuits described but I haven't seen a diagram of what that looks likes.  There was a post about a year ago where a GI took a bunch of photos of an original M231 FPW so that should show it better.

The striker is the M231 FPW system.

I am probably wrong but I don't think the striker slides back and forth. I think the striker stays in a fixed position within the carrier and just makes contact with the firing pin when the bolt cams into the locked position causing the cartridge to fire.

ETA - I am refering to the FPW not the LMG.

wouldn't it be kind of hard to achieve slam fired ignition in a rifle cartridge with a rotating bolt?

you couldn't fire the cartridge until the bolt stopped turning, and it's not going to move forward once it's locked into the fully closed position

subguns that operate on blowback are pretty sloppy mechanism, they have a massive bolt and spring just for relatively light rounds like a 9mm and they still bulge brass sometimes

the M16 mechanism is pretty much exactly the opposite, its a precision gas operated locked-breech turnbolt design

The carrier on the AR/M series moves forward only during the last 1/16" or so before the carrier is fully into battery. So yes it is possible.

All you could ever want to know about the M231 FPW: http://www.ar15.com/forums/t_3_123/459672_M231_Firing_Port_Weapon_Review________56K_MASSACRE______________.html

Yes seen that thread but that trigger mechanism doesnt match the pic of the trigger mechanism that I posted in the OP

Also does anyone make reproduction M231 FPW parts for a closed bolt semi auto retro build?

Check the retro forum.  At one time, there was a guy making repro stocks and receiver extensions, but they just worked with the standard buffer, BCG and FCG, internally.  Same for repro stocks and handguards.  Not sure if anyone has made repro sideplates or longer faux-fat pins, though, and to be really correct, you'd need to notch your lower for the sideplate.

For what it's worth, your OP pic is also not the Tactical Innovations AM-15 open-bolt setup, which looks to be a slightly-modified version of the Colt LMG system, in that the hammer holds the bolt back, and a spur on the trigger operates a hinged auto sear to first release the bolt carrier, but catch the hammer on the sear, then release the hammer when the bolt is in battery.

it's not a matter of the carrier, the firing pin is set into the bolt face.  the bolt doesn't move forward once it's locked into place and you cannot begin ignition until it's locked.

blowback guns operate on momentum, the ignition of the cartridge generally starts before the bolt is fully in battery, but the huge mass and energy of the bolt keep the ignition from retarding the bolt.

Imagine a 5.56 round ignition and that tiny lightweight bolt trying to shove the thing into battery as the case obfurscates.  And also spin into lock up with the lugs.

I'd have to see that to believe it.

I've had 9mm rounds cook off in the barrel of my MAC 10, the gun laughed at it.  Try firing a 50,000 PSI 223 round even slightly out of battery, it'll destroy it.   The M1 garand is probably the strongest military rifle ever made, an OOB round will pop it apart like a hand grenade.

The M231 uses a standard bolt and firing pin.

Ignition is achieved by a sliding weight with an arm/shaft in the bolt carrier that hits the firing pin shortly after the bolt carrier goes into battery.

ah so

that would work


the biggest problem with open bolt guns is mentally keeping yourself from "dropping the bolt" on a round to chamber it like you do on closed bolt guns.

It took me a while to get used to that, for safety sake  i don't even insert a mag until I'm ready to shoot.

They won't sell them. I've tried a couple of times.

I would love to get a chance to fire the M231 FPW setup with one of my good blaster uppers that I know groups well.
The advanced ROF would be interesting.

Isnt the difference in the LMG and M231 the ROF. Its got to be about the stocks. Nestled springs VS a hydraulic bolt.
Id be willing to bet that the trigger mechanisms are different for a reason.
The repro m231 stock would be a better match for the LMG bolt, being it uses a standard carrier

Dont some claim open bolt m16's that arent colt made tend to egg the hell out of triggers?
steel side plates might help that.

I want an open bolt setup. Cant explain why.

I have an upper but without a modified FA lower it does nothing special. I'm not sure why they wouldn't sell complete guns to LE unless they aren't tooled up for production. Their current focus is on the new 6.8 architecture.